Operatives working for the German forces either set fires or detonated small bombs to destroy ammunition being stored at the pier that was to be used against Germany by U.S. allies in WWI. The largest explosion at 2am was the equivalent of a 5 to 5.5 magnitude earthquake, shattered windows across Jersey City and Lower Manhattan, and famously damaged the Statue of Liberty, causing tours of the torch to be prohibited still to this day. Read more from the FBI's history of the explosion